1. Technical Field
The present invention relates to methods of ethanol fermentation. More specifically the present invention relates to processing stillage.
2. Background Art
Throughout this application, various publications, including United States patents, are referenced by author and year and patents by number. Full citations for the publications are listed below. The disclosures of these publications and patents in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
Ethanol fermentation is the biological process by which sugars are converted into ethanol and carbon dioxide through yeast fermentation. Corn is one of the main feedstock materials used to produce ethanol. Dry milling has previously been used to produce ethanol from corn on other starch sources through fermentation (shown generally in FIG. 1, labeled “Prior Art”). Corn is milled to flour, slurried, and treated with enzymes to convert the starch to sugars. The sugars are converted to ethanol in large fermenters. The ethanol is recovered through a distillation process. The residual spent grains, referred to as whole stillage, contains corn germ, corn bran, corn oil, unconverted starch, unfermented sugars, yeast cells, yeast metabolites, and other suspended and dissolved solids. The whole stillage stream is generally separated into wet distillers grain (WDG) and thin stillage. The wet distillers grains can be dried to produce Dry Distillers Grain (DDG). A portion of the thin stillage, referred to as backset, is recycled back to the front end of the ethanol process as make up water. The remaining thin stillage is evaporated to syrup, added to the wet distiller's grains and dried as Dried Distillers Grains with Solubles (DDGS). WDG, DDG, and DDGS are important co-products that are critical to the economic viability of the ethanol process. However, their value can be enhanced by extracting more valuable co-products from these streams. It has only recently been a goal to recover additional materials from the co-products for further use.
Materials, such as oil, protein, and other solubles in the whole stillage are very valuable; however, recovery has shown to be inefficient and uneconomical. Recently, various methods have been attempted to recover the additional materials from stillage. These methods include traditional separation techniques such as heating the stillage stream and performing evaporation, using centrifugation, or using membrane filtration, in order to recover these additional materials. The result of each of these separation processes on stillage is a concentrate and a water phase wherein most of the solids have been removed.
A number of methods have been developed involving heat treated stillage for the recovery of fermentation by-products, especially oil. U.S. Patent Application Publication No. 2009/0250412 and U.S. Pat. No. 7,608,729 to Winsness, et al. disclose methods for recovering oil from stillage concentrate including oil resulting from a process used for producing ethanol from corn. Winsness, et al. generally believe that filtration increases operating costs and therefore focus on separation by heating. In one embodiment, the method includes heating the stillage concentrate to a temperature sufficient to at least partially separate, or unbind the oil. The heating step includes heating to a temperature above 212 degrees F. but less than about 250 degrees F. The method also includes the step of pressurizing the heated stillage concentrate to prevent boiling. The method further includes recovering the oil from the treated stillage concentrate using a gravity separation process including centrifugation. The process disclosed by Winsness, et al. does not include treatment of unconcentrated stillage streams. While oil can be recovered from the method of Winsness, et al., there are many products in the thin stillage that are not recovered. For example, the process disclosed by Winsness et al. does not include recovery of a high solids-high protein fraction and a stickwater fraction (as defined below) nor the improved fermentative value and alternative uses of stickwater. Furthermore, it is generally accepted in the art that heating the thin stillage to higher than 250 degrees F. is harmful to proteins and other biological components.
U.S. Pat. No. 6,106,673 to Walker discloses a process and system for the separation of a fermentation process byproduct into its constituent components and for the subsequent recovery of those constituent components. The process requires 1) mixing a starting mixture containing ethanol byproducts with a liquid (water) to form a diluted mixture, 2) heating of the diluted mixture containing the byproducts so as to separate the oil from a base component (fiber) of the byproduct to which the oil is bound at a temperature from about 140 degrees F. to about 250 degrees F., followed by 3) recovering oil, the base product (fiber), and possibly other substances such as molasses from the mixture. The process can be performed on a large scale and in a continuous fashion using a mechanical separator to recover fibers from the diluted heated mixture to produce a solids stream and a liquor stream and by then removing oil and insoluble substances from the liquor stream in an evaporator assembly. Energy consumption and water consumption are minimized through 1) the use of waste heat from the system's dryer as an energy source for the evaporator assembly and 2) the use of condensed liquids from the evaporator assembly to dilute the mixture. There is no disclosure in Walker '673 of recycle of recovered water or stickwater to fermentation or improvement of fermentation rate or yield by recycle of any or the entire liquor stream to upstream operations.
Thus, while heating and mechanical separation described in prior art provides some separation of co-products, especially oil, it was not recognized that the use of all or a portion of hydrothermally treated stillage or stickwater can improve fermentation processes.
Thermal hydrolysis has been investigated as a pretreatment step prior to anaerobic digestion of biomass, in particular the anaerobic digestion of waste activated sludge from biological waste water treatment facilities and the pretreatment of cellulosic biomass prior to enzymatic hydrolysis to liberate cellulosic sugars. The former has been commercially implemented while the latter remains a research and development endeavor. Camacho, et al. (Proceedings of the WEFTEC® 2008 Conference, Chicago, Ill. Water Environment Federation) reviewed the use of thermal hydrolysis as a pretreatment to anaerobic digestion of activated sludge and noted the improvements in both sludge dewaterability and biogas yield during anaerobic digestion. Optimal treatment temperatures were generally in the range of 150-200° C. (302-392° F.).
Yu, et al. (Energy & Fuels 2008, 22, 46-60) reviewed the use of hot compressed water (HCW) as a pretreatment for biomass in the production of cellulosic biofuels. The authors focused on the unique physicochemical properties of HCW and the chemistries imparted by HCW as well as the yield of fermentable sugars resulting from enzymatic hydrolysis of the pretreated biomass.
Kim, et al. (including Ladisch) (Bioresource Technology 2008, 99, 5206-5215.) investigated the thermal hydrolysis of distiller's dry grains and solubles (DDGS) from a dry grind ethanol facility as a cellulosic pretreatment prior to enzymatic hydrolysis of the cellulosic biomass. The objective of the thermal treatment of Kim, et al. was to prepare the cellulose of DDGS for downstream enzymatic hydrolysis to glucose by cellulase and beta-glucosidase enzymes. U.S. Pat. No. 5,846,787 to Ladisch, et al. discloses use of thermal hydrolysis in the range of 160-220 degrees C. (320-428 degrees F.) as a pretreatment for cellulosic biomass prior to enzymatic treatment with cellulase.
Other efforts have involved heat treatment and filtration of depleted lignocelluosic fermentation hydrolysate broth to separate undissolved solids from the liquid phase and create a low solids liquid (Hennessey, et al., U.S. Patent Application Publication No. 2012/0178976 and Hennessey, et al., U.S. Patent Application Publication No. 2012/0102823, assigned to Dupont).
It is recognized that the temperatures utilized for hydrothermal pretreatment of biomass prior to cellulosic ethanol fermentation and municipal waste prior to anaerobic digestion are greater (300 degrees F.-450 degrees F.) than those preferred for treating stillage in the present invention (220 degrees F.-300 degrees F.).
Stillage has been investigated for enhancing biological processes. For example, in the prior art ethanol process of FIG. 1, stillage is recycled to the front end as make-up water in the slurry and is referred to as “backset”. The proteins and nutrients in the stillage have been recognized as aiding fermentation; however, this benefit is marginal and the suspended solids in backset limit the amount of fresh grain solids that can be added to fermentation. Therefore, there is a need for treating stillage to increase its value in fermentation and other biological processes.
A number of biological and non-biological methods have been developed for the improvement of thin stillage. Jacob P. Tewalt, et al. in WO2012/122393 assigned to POET Research Inc. disclose a method to clarify thin or whole stillage by growing fungi. Wicking, et al. in U.S. Patent Application Publication No. 2012/2094981 assigned to North American Protein Inc. disclose the use of fungi to remove inhibitory compounds from stillage and create a treated backset having improved ethanol fermentation performance.
J. Van Leeuwen, et al in U.S. Patent Application Publication No. 2010/0196994 assigned to Iowa State University disclose a method of continuous fungi cultivation on thin stillage to produce useful products and remediated water with significantly reduced COD (chemical oxygen demand).
M. Reaney, et al. in U.S. Patent Application Publication No. 2011/0130586, assigned to the University of Saskatchewan, disclose a method of recovering a recyclable water from thin stillage or dewatered (concentrated) thin stillage by polar solvent and/or oil extraction of microbial inhibiting metabolites such as glycerol, lactic acid and 2-phenylethanol (PEA) and the phospholipid α-glycerylphosphorylcholine (GPC) which has potential value in pharmaceutical applications.
J. Jump, et al. in U.S. Pat. No. 7,641,928, assigned to Novozymes North America Inc., disclose the use of enzymes to treat stillage and improve the dewatering properties of stillage.
Prior art processes have tried to remove suspended solids from thin stillage with various flocculating, coagulating or precipitating additives and chemical agents. J. Hughes, et al., in U.S. Pat. No. 8,067,193, assigned to Ciba Specialty Chemicals, discloses the use of anionic polymer additives to increase coagulation and precipitation. D. W. Scheimann and A. S. Kowalski in U.S. Patent Application Publication No. 2006/0006116 assigned to Nalco Company, disclose methods of coagulating and flocculating thin stillage suspended solids using anionic polymer flocculants, cationic coagulants and microparticulate settling aids and removing said suspended solids from the thin stillage. J. Collins, et al. in U.S. Patent Application Publication No. 2012/125859, also assigned to Nalco Co., disclose a method involving ionic flocculants for conditioning and processing whole or thin stillage to aid in the separation and recovery of protein and oil fractions. C. Griffiths in U.S. Patent Application Publication No. 2007/0036881 assigned to Ciba Specialty Chemicals, discloses the removal of suspended solids from thin stillage by treatment with polyacrylamide and electrocoagulation. Verkade, et. al. in U.S. Patent Application Publication No. 2009/0110772 assigned to Iowa State University, describe separating solids from a processed broth through chemical reaction with a phosphorous oxoacid to increase the water solubility of insoluble cellulosic, melaninic, ligninic, or chitinic solids.
Various filtration, microfiltration and ultrafiltration processes have been disclosed in the prior art. Bento, et al. in U.S. Pat. No. 5,250,182 assigned to Zenon Environmental Inc., disclose a step-wise membrane separation process to recover lactic acid and glycerol together, from thin stillage in an ethanol stream. The stepwise process consists of ultrafiltration (UF), nanofiltration (NF) and reverse osmosis membrane units. Demineralized water may be recycled to fermentation or to boiler water make-up feed. Bento et al. suggest that the use of the membrane separation process in the production of ethanol based on the dry-milling of corn, substantially reduces or eliminates the use of a conventional evaporator
Other prior art processes have described removal of solids from the clarified aqueous phase through the use of filters after separation of hot (140-212 degrees F.) concentrated thin stillage into a light oil phase and a heavy aqueous phase and treating the oil phase with alkali chemicals including spent clean in place (CIP) solutions (Woods, et al., U.S. Patent Application Publication No. 2011/0275845, assigned to Primafuel).
None of these biological and non-biological prior art methods for treatment of stillage and solid-liquid separation (with or without benefit of additives) has been shown to improve fermentation by the surprisingly simple process of hydrothermally treating stillage and utilizing the treated stillage as a media component in a fermentation process.
Various methods have been proposed for utilizing stillage for biological purposes other than ethanol fermentation. M. Kriesler and D. Winsness in U.S. Patent Application Publication No. 2010/0028484 assigned to GS Cleantech, disclose methods for producing lipids from various stillage streams by the yeast Rhodotorula glutinis. Kriesler and Winsness also disclose conditioning of the stillage feed stocks by various pre-treatment methods including steam explosion, autohydrolysis, ammonia fiber explosion, acid hydrolysis, sonication and combinations thereof prior to inoculation with the lipid producing micro-organism.
M. Ringpfeil in U.S. Pat. No. 5,981,233 assigned to Roche Vitamins Inc. discloses a process for manufacturing a xylanase enzyme complex from pre-treated thin stillage of rye, where the pretreatment includes removing solids from the rye thin stillage, evaporation of water, adding other nutrient components and autoclaving prior to culturing the enzyme producing organism (Trichoderma).
In summary of the prior art, methods for improving ethanol fermentation, fermentation of other products, or growth of non-alcohol producing microorganisms by addition of stillage which has been hydrothermally treated in the preferred range of 220 degrees F.-300 degrees F. of the present invention has not been described in patents or literature. It has been discovered for the first time that hydrothermally treating stillage and adding the treated stillage to a fermentation process increases fermentation rates and titers. Therefore, it is shown herein that the present invention provides a simple method for improving fermentation by the addition of hydrothermally treated stillage.
While heating and filtration described in prior art provides some separation of co-products, recovery is limited and costs remain high. One advantage of the present invention is that hydrothermal fractionation of stillage produces a physicochemical alteration, which enables a facile separation allowing for improved recovery of co-products. With respect to the present invention, “hydrothermal fractionation” means heating a substantially aqueous stillage stream to a temperature within a prescribed temperature range, and holding at temperature for a period of time within a prescribed residence time range. A saturation pressure is established and maintained during the hydrothermal fractionation step. Physicochemical alteration means that both physical and chemical changes are imparted to the stillage by the hydrothermal fractionation step. Manifest physical changes include changes in the rate of phase separation, relative phase volumetric fractions and phase densities, phase hydrophobicity and changes in color or appearance. Chemical changes include changes in the distribution of non-soluble protein, fat (oil) and carbohydrate (fiber) between the substantially liquid phase and the substantially solids phase. Other chemical changes include solubilization and/or hydrolysis of components to increase the levels bio-available protein and ammonia in the soluble phase. These physical and chemical changes are mutually dependent and hence the term physicochemical is applied.
Thus heating of stillage has been performed as described in the prior art for recovery of corn oil and other by-products; however, it was not recognized that the hydrothermal treatment of stillage according to the present invention imparts physicochemical changes enabling facile separation into a low solids stickwater fraction, oil and high protein solids fraction. Furthermore and importantly, it will be shown herein that the low solids stickwater fraction provides an enhanced nutrient medium for ethanol and other fermentation processes, thus providing an economic advantage.
Therefore, there is a need for a simple method of producing a physicochemical alteration that changes the co-products in stillage and enables facile separation of co-products in ethanol processing as well as providing streams suitable for improving biological production and recovery of valuable co-products, extracts, metabolites and treated water.